An archaic or dialectal term, possibly referring to a type of plant, alder tree, or variant spelling of avena (oats).
Possibly from Latin avena (oats) or related to alder trees. The exact meaning is unclear; the word appears sporadically in Middle English texts without clear definition.
Medieval people had dozens of names for common plants—some based on Latin, some on local languages—and many of those names just disappeared when standardized naming took over. 'Avern' might be a ghostly reminder of how differently people in different villages talked about the exact same plant.
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